`BarrierOp` and `Circuit._add_conditional_barrier` #4230

Job	Run time
Check tket library version	26s
Check documentation build	1m 27s
Build and test (tket) (windows-2022)	56m 42s
Build and test (tket) (ubuntu-22.04)	32m 42s
Build and test (tket) (macos-12)	27m 53s
Check C++ code formatting	54s
Build and test pytket (ubuntu)	11m 7s
Build and test (tket)	0s
Build and test (tket)	0s
Build and test pytket (macos)	12m 33s
Build and test pytket (macos-arm64)	4m 22s
Build and test pytket (windows)	37m 22s
Build and test tket (macos-arm64)	4m 0s
Check pytket line and branch coverage	5s
Publish pytket coverage	0s
	3h 9m 33s

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